See-through head mounted displays allow a user to view a displayed image or a see-through view of the scene in front of the user. See-through head mounted displays can also allow the user to view a combined image comprised of a displayed image and a see-through view of the scene in front of the user in which the displayed image is overlaid on the see-through view. In different modes of operation, the see-through head mounted display can present the displayed image so that the area of the displayed image is transparent, semitransparent or opaque. In the transparent mode, the see-through view of the scene is unblocked and an overlaid displayed image can be provided with low contrast. In the semitransparent mode, the see-through view of the scene is partially blocked and an overlaid displayed image can be provided with higher contrast. In the opaque mode, the se-through view of the scene is fully blocked and an overlaid displayed image can be provided with high contrast.
Alternatively, some head-mounted displays provide a see-through display for an augmented reality view in which real-world scenes are visible to a user but additional image information is overlaid on the real-world scenes. Such an augmented reality view is provided by helmet mounted see-through displays found in military applications and by heads-up displays (HUDs) in the windshields of automobiles. In this case, there can be multiple areas for displaying images over the see-through view.
U.S. Pat. No. 5,151,722 describes a head mounted display with a folded optical path and a beam splitter with a curved mirror to present an image from a CRT image source to the user's eye. The image source is positioned on the side of the user's head with the optical path multiply folded to present the image light from the image source into the user's eye. The curved mirror is positioned between the user's eye and the scene in front of the user. A see-through version of the head mounted display is also discussed wherein the curved mirror is a partially reflective mirror so the user can see through the curved mirror to view the scene in front of the user. However, the CRT image source is large and heavy so that it is not well suited for head mounted displays. The multiply folded geometry with the image source located on the side of the user's head makes for a larger beam splitter and a thicker geometry in front of the user's eye so that the overall size of the head mounted display is larger. Image light that passes through the partially reflecting mirror is uncontrolled and as such, a portion of the image light escapes through the front of the see-through head mounted display and is seen externally as eyeglow.
U.S. Pat. No. 5,699,194 discloses a see-through head mounted display with a waveguide wherein the outer surface is a partially reflecting mirror. In this see-through head mounted display, image light from the image source is reflected multiple times from different areas of the partially reflecting mirror before the image light is presented to the user's eye. In addition, a corrective lens is provided so that distortions of the see-through view of the scene are reduced. And, a liquid crystal shutter is provided to block incoming light from the scene so that the see-through headmounted display can be operated in an opaque mode. Image light that passes through the partially reflecting mirror is uncontrolled and as such, a portion of the image light escapes through the front of the see-through head mounted display and is seen externally as eyeglow.
U.S. Pat. No. 6,693,749 describes a head mounted display with a polarizing beam splitter cube to reduce light losses and improve efficiency. An image source is positioned above the user's eye and the optical path is folded once to present the image light from the image source to the user's eye. A curved fully reflecting mirror is positioned below the user's eye to focus the image light at the user's eye. An unpolarized image source is used so that half of the image light from the image source passes through the polarizing beam splitter while the other half of the light is reflected away from the user's eye and toward the scene in front of the user. A polarizer is positioned in front of the polarizing beam splitter cube to block the half of the light that is reflected away from the user's eye and thereby make the user less observable by others in the neighboring environment. However, the polarizing beam splitter cube is large and heavy, so that it is not well suited for use in a head mounted display. Further, the curved mirror is also relatively large so that by locating the mirror below the user's eye, the thickness of the see-through head mounted display is necessarily larger.
There is a need, therefore, for an improved see-through head-mounted display that provides a thinner, lighter weight display which also controls escaping light to reduce eyeglow.